1. Field of the Invention
The present invention relates to a diffractive optical element, an optical system, and an optical apparatus.
2. Description of the Related Art
A diffractive optical element of a blazed structure is configured to highly efficiently diffract light of one specific order (or designed order) and a specific wavelength. For sufficiently high diffractive efficiency of the specific order in an overall visible wavelength range, Japanese Patent No. 3,717,555 discloses two diffractive gratings adhered to each other. Each diffractive grating is made of a low refractive index high dispersion material and a high refractive index low dispersion material. A height of the diffractive grating is properly set. The diffractive optical element of this type will be referred to as an “adhesion two-layer DOE” hereinafter. For high diffractive efficiency equal to or higher than 99% in the overall visible wavelength range, Japanese Patent Laid-Open No. 2008-241734 uses a material which has a partial dispersion ratio θgF smaller than that of a usual material (or a linear abnormal dispersion).
However, due to the behavior of the wall surface section that provides no diffractive action, the wavelength characteristic of the diffractive efficiency of the diffracted light of the designed order reduces on the long wavelength side, and the diffractive efficiency of the red wavelength range becomes lower than that of the blue wavelength range. Unnecessarily diffracted light other than the designed order is likely to stand out on the red wavelength range. If the red wavelength range is relatively intensified by using an antireflective film, image processing, etc. for color balancing of the diffracted light of the designed order, unnecessary light in the red wavelength is also intensified and highlighted.